Direct Broadcast Satellite television transmission receivers comprise a Low Noise Block Downconverter (LNB) mounted on an antenna or satellite dish connected, via coaxial cable, to an indoor tuner attached to a television set or VCR. The LNB is connected to the antenna or dish and converts the satellite signal received by the antenna to a frequency and signal level suitable for processing by the tuner and television or VCR electronics.
The Low Noise Block Downconverter assembly typically includes a monolithic microwave integrated circuit (MMIC) downconverter mounted on a printed circuit board along with support circuitry, additional amplifier stages and filters to provide increased amplification and reduced front end noise. Typically, the LNB Downconverter receives microwave frequencies of approximately 11 GHz to 12 GHz and first amplifies the signals through several high electron mobility transistors (HEMTs) and two amplifier stages in the MMIC. The MMIC also downconverts the RF signals to an intermediate frequency (IF) of approximately 1000 MHz in a mixer, then amplifies the IF signal.
The Direct Broadcast Satellite television market is a large consumer market with sales of many millions of units each year. The market is price sensitive with competitive devices offered by several companies. Reduction of the LNB Downconverter product price is important for a manufacturer to remain competitive in the market. Product price reduction can be achieved through reduction of the component costs, especially the MMIC.
One design approach, as shown in FIG. 3, is to bias the mixer through a voltage dropping resistor that is separate from the amplification circuitry as discussed in the paper "A High-Performance, Miniaturized X-Band Active Mixer for DBS Receiver Application with On-Chip IF Noise Filter" published in the IEEE Transactions on Microwave Theory and Techniques, Vol. 38, No. 9, dated September 1990. This approach yields a mixer bias voltage that is sensitive to DC bias current and additional power dissipation in the bias resistor.
Another design, as shown in FIG. 4, utilizes an active load to bias the mixer which provides a stable mixer bias voltage. The active load circuit does not act as an intermediate frequency (IF) amplifier, requiring additional circuitry and increased power dissipation.
Another configuration, as illustrated in FIG. 5, connects the local oscillator amplifier in parallel with the mixer so that the sources and drains are common. There is no known teaching of how to bias the mixer. Assuming the bias is provided through the IF filter, there is no suggestion that the bias voltage is provided by the IF amplifier circuit.